Y Y Connected VT Grounding 1

[3winding]

I am installing (3) 2400/120V VT's which will be Y connected, for metering, and have a concern about grounding. Is it good practice to ground the Y point of both high and low voltage windings? Is their a concern of an alternate zero sequence path with the HV grounded? Will leaving the HV Y point float present any problems - metering/relaying? I have seen both configurations used. The system is on an IPP generator (Delta Connected Induction) with a Y-Y step up transformer.

 

[scottf]

You must ground the HV Y point...you don't have any choice in that matter.

If you connect the secondaries in Y, then you should ground that point as well....and only in one point.

I've never heard about a concern for zero-sequence path by ground HV side of VTs connected in Y. If the VTs are single-bushing units, they must be grounded.

 

[busbar]

The primary wyepoint should be solidly grounded to limit core saturation/overvoltage on the primary windings from secondary-burden imbalance (for a 4160V bus.) ANSI Std C57.13 §7 is an applicable reference.

 

[busbar]

I think the zero-sequence path would only be a problem if the secondaries were delta connected. pintalo implies grounded-wye secondaries, correct?

 

[jlazucena]

Previous Posts from Scottf and Busbar already defined that you need to ground both sides.

For IEEE recommended practice for harmonic control you may check STD519.
Now, the circulation of harmonics in this application is usually not a concern because metering and relaying equipment will not import nor produce harmonics; unless that you are are in a harmonic rich environment (With distortion factors >20%). Or some distortion and using induction type meters.
See Chapter 6 of STD 519.
In power system you may have to evaluate wye-grd versus ungrounded to diminish the circulation of triple harmonics or to reduce short circuit levels, but that is discussion for another Post. Have a good weekend.

 

[3winding]

Thanks everyone for your replies.

 

[jbartos]

Suggestion: Grounded wye primary winding and grounded wye secondary winding will provide a path for the zero sequence current. Reference: Westinghouse T&D Reference Book. The zero sequence current tends to be eliminated by interrupting its path, i.e. by a suitable transformer winding connection on its primary side and secondary side and by the proper grounding.

 

[cuky2000]

Check the enclose site. There are different connection configurations with pros & cons that could help to clarify your concerns.

Good luck

http://www.beckwithelectric.com/lc/protection/vtcongnd/summary.htm

 

[asymptote]

You must also consider the primary souce system earthing in conjunction with parallel operation and the consequnce of inverting via the VT's. In the UK it is not normal to earth the HV wdg of a Yy0 VT but it is common practice to earth the LV yellow and not the star pt.
GRW